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An ATtiny85 can automate watering by reading a soil-moisture sensor and switching a separate low-voltage pump through a properly rated MOSFET or transistor. The documented Hackster build also waits after each watering and checks again, helping prevent repeated pump cycles while water spreads through the pot. It is a hobby project, not a proven unattended-care appliance: reliable operation depends on sensor placement and calibration, pump timing, and safeguards for faults such as an empty reservoir.
How the ATtiny85 watering system works
The controller samples a soil-moisture sensor through an analog input. Its sketch maps a selected range of sensor readings to a displayed 0–100 value, then compares that mapped reading with a configurable threshold. If the reading crosses the threshold, the ATtiny85 controls a switching device that powers the pump. The number is a project-specific mapped reading, not a universal percentage of water in soil.
After a pump pulse, the logic waits before taking another decision. That pause matters: water does not instantly spread evenly through a pot, so an immediate second reading could prompt unnecessary watering. The Hackster author describes limiting repeat watering to avoid overflow while the soil responds. The project page contains multiple code versions with different thresholds, intervals, and limits; settings from one version should not be assumed to describe another. See the project and its code versions.
Parts in the documented Hackster build
The exact-title project was published on May 13, 2018. Its parts list and description identify the following components. These are details of that build, not a universal bill of materials or a recommendation that the parts are electrically interchangeable.
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| Part | Documented role or specification |
|---|---|
| ATtiny85 module | Reads the sensor and controls the pump-switch input. |
| SparkFun soil-moisture sensor | Provides an analog reading; the sketch powers it for measurements. |
| Pololu Mini MOSFET Slide Switch with reverse-voltage protection | Switches the pump load; the microcontroller pin does not power the motor directly. |
| Water pump | Listed as 12 V, 280 L/h. This is the component specification in the parts list, not an independently verified flow measurement. |
| Power adapter | The author describes powering the system with a 9 V, 0.5 A adapter. The pump and system supply details belong to this build; check the ratings and wiring for the specific parts selected. |
| Display | An 8×2 monochrome LCD with an I2C adapter. The author notes that another display can be used. |
| Reservoir and tubing | The pump is submerged in a 5-liter bottle and feeds water to the pot. |
Before wiring, check the exact board’s pin labels, operating voltage, and configuration against its documentation and the Microchip ATtiny25/45/85 datasheet. Confirm that the switching stage and supply are rated for the pump’s voltage and current. A pump is a motor load: do not connect it directly to an ATtiny85 I/O pin.
Wiring and design decisions to check
Separate control from motor power
Use the ATtiny85 output to control a suitably rated MOSFET or transistor stage, and power the pump from an appropriate supply. The Hackster parts list uses a MOSFET switch; a separate ATtiny85 build by Mikael Kunnari uses an NPN transistor. Those examples illustrate the switching-stage concept, not a drop-in wiring diagram. Verify the chosen device’s current and voltage ratings, the board and sensor voltage requirements, and the pump’s supply before assembling the circuit.
Choose how the sensor is powered
Resistive moisture probes can corrode when left energized. In Kunnari’s design, the controller powers the sensor briefly during sampling to limit corrosion. The Hackster sketch also assigns a controller pin to sensor power. Follow the requirements of the exact sensor and circuit rather than assuming every sensor should be wired or powered the same way. Kunnari’s ATtiny85 watering build uses a FunDuino sensor, an aquarium pump, a 12-to-5 V converter, a transistor, water-level sensing wires, and an opaque tank of about 5 liters.
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Plan for water and faults
Decide how the system will behave if the reservoir empties, the sensor becomes disconnected or gives a stuck reading, or water fails to reach the pot. The cited builds demonstrate water-level sensing in Kunnari’s design and repeat-cycle limits in the Hackster project; neither establishes that every failure mode is covered by the other. A cycle limit can reduce the chance of overflow, but it does not prove that the plant received water or that the sensor is healthy.
Calibrate the reading and position the sensor
The displayed value depends on the sensor, soil, and code mapping. It is not a standardized soil-moisture measurement, and the Hackster author says readings vary between sensors and recommends recalibration. The page describes recording readings with the sensor in air and with it short-circuited, then adjusting the code mapping and threshold. Use the project’s method only with an understanding of the sensor and circuit, and make calibration decisions for the actual pot and soil rather than treating a posted threshold as a plant-care rule.
Placement affects what the controller sees. The author warns that a local wet patch can mislead a sensor: mud adhering to the probe may keep its reading wet even when other soil is dry. Conversely, if the probe is too far from the outlet, water may take a long time to reach it. Position the sensor near enough to the outlet to respond to watering, while avoiding a placement that only measures a persistently soaked spot. Observe how the actual pot responds before relying on automatic cycles.
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Recheck the calibration periodically and after replacing the probe, changing the soil, or changing the pot or watering layout. A threshold is meaningful only in relation to that sensor’s readings and the specific setup.
Set watering and recheck behavior cautiously
The first posted Hackster code version uses a threshold of 95 and a 15-second pump pulse. In the project narrative, the author says they later changed the threshold to 75 for a Mimosa pudica after replacing the sensor. These are examples from one project, not recommended thresholds or watering durations for other plants.
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What the documented results do—and do not—show
The Hackster author reports that a 5-liter reservoir lasted roughly one to two months for their small pot. The author also reports more than 18 months of operation before the original sensor failed; after replacement, they adjusted the threshold from 95 to 75. These are single-build anecdotes, not expected reservoir duration, sensor lifespan, or guarantees for other plants and installations.
The project code comments report 26 mA while the CPU, sensor, MOSFET, and display are active, 20 mA between readings, and 8 mA in deep sleep with the display on. Those are figures reported in code comments for the shown setup, not independent measurements or a validated estimate of total system energy use.
For comparison, Kunnari’s separate design takes hourly readings, uses an aquarium pump and an opaque tank of about 5 liters, briefly powers its resistive sensor, and switches the motor through a transistor. The two projects demonstrate different implementation choices; neither establishes a universally best sampling schedule, pump, or tank size.
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- Through adjusting the potentiometer to control the soil humidity threshold,can automatic watering the vegetable garden, garden, control the flower pot soil moisture.
- Include Relay module, Mini water pump, Plastic Battery Storage Case Holder, USB power cable, Jumper wires, 0.5m Vinyl Tubing and Soil Moisture Detector Module.
- Note:Do not pour water directly on the sensor. The humidity of the water will rust the sensor.
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- Soil Moisture Sensor Kit Automatic Watering System Manager with Mini Water Pump for Arduino DIY Kit
When this project is a sensible fit
An ATtiny85 watering build is a reasonable hobby project if you want to learn analog sensing and motor switching, can calibrate and observe the setup, and can provide a safe water-and-electronics arrangement. It is less suitable as a set-and-forget substitute for checking a plant: sensor drift, local wetness, a dry reservoir, blocked tubing, or a failed pump can all undermine the decision to water. Treat autonomy as a control feature that needs testing and maintenance, not a guarantee of plant care.
For further design investigation, markfickett’s Arduino-based plant waterer is a separate project; it is not an ATtiny85 wiring or compatibility reference.
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